The present application is generally related to steering tools for horizontal directional drilling and, more particularly, to a system and method for multimode use of steering information and homing information.
A boring tool is well-known as a steerable drill head that can carry sensors, transmitters and associated electronics. The boring tool is usually controlled through a drill string that is extendable from a drill rig. The drill string is most often formed of drill pipe sections, which may be referred to hereinafter as drill rods, that are selectively attachable with one another for purposes of advancing and retracting the drill string. Steering is often accomplished using a beveled face on the drill head. Advancing the drill string while rotating should result in the drill head traveling straight forward, whereas advancing the drill string with the bevel oriented at some fixed angle will result in deflecting the drill head in some direction.
One approach that has been taken by the prior art, for purposes of monitoring the progress of a boring tool in the field of horizontal directional drilling, resides in what is commonly referred to as a “steering tool”. This term has come to describe an overall system which essentially predicts the position of the boring tool, as it is advanced through the ground using a drill string, such that the boring tool can be steered along a planned drill path within the ground. Steering tool systems are considered as being distinct from other types of locating systems used in horizontal directional drilling at least for the reason that the position of the boring tool is monitored in a step-wise fashion as the boring tool progresses through the ground. For each position of the boring tool, pitch and yaw angles of the drill-head can be measured in coordination with extension of the drill string. From this, the boring tool position coordinates are obtained by numerical integration. Nominal or measured drill rod lengths can serve as a step size during integration. For this reason, positional error can accumulate with increasing progress through the ground. The positional error can be attributed, at least in part, to pitch and yaw measurement errors as well as underground disturbances of the earth's magnetic field, which can cause yaw measurement bias errors. Thus, the boring tool can arrive at a considerable offset from the endpoint target of the bore plan.
A homing system, in contrast, relies on an electromagnetic signal that is emitted from the boring tool. The electromagnetic signal is received at a receiving position for generating homing commands that are used in guiding the boring tool to a target in relation to the receiving position. It should be appreciated that, in a homing system, numerical integration of orientation parameters of the boring tool is not necessary, therefore eliminating concerns with respect to accumulating positional offset errors. One example of an advanced homing system is represented by U.S. Pat. No. 6,727,704, which is commonly owned with the present application and hereby incorporated by reference. Applicants recognize, however, that the range of the electromagnetic signal from the boring tool to the receiving position can be significantly shorter than the length of an intended drill path.
Another form of prior art system for monitoring a boring tool uses what is often referred to as a walkover locator. In such a system, an operator carries the walkover locator above the surface of the ground for receiving the electromagnetic signal. The location of the boring tool can be established, at least in part, based on the ability of the operator to change the positional relationship between the walkover locator and the boring tool. In this way, various field-defined points can be identified at the surface of the ground which characterize the electromagnetic signal. Applicants recognize that in some instances the use of a walkover locator is impractical. For example, the drill path can extend under a busy highway, river, lake or other such obstacle.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.